Linux – CentOS – hardware raid 1 of two drives not being treated as single drive by fdisk

centoslinuxraid1

I am a bit new to hardware configuration in general, much less with linux, so please give me a little slack 🙂

I have the following setup: 1 320GB Drive that contains the OS (centOS) and 2 250GB drives that have a hardware RAID1 configuration (accomplished via my ASUS motherboard raid utility, they are mirrored as one device)

The problem is that everywhere I am reading online, the centOS is supposed to see the 2 disks that have RAID1 as a single device. However, when I run fdisk -l I am shown the three disks individually, the 320GB drive (/dev/sdc) and the individual 250GB drives (/dev/sda) and (/dev/sdb)

I was under the impression that I was supposed to see two, the /dev/sda (OS drive) and some kind of 'raid' device (/dev/sdb) that I can select for partitioning and then mounting.

What am I doing wrong? Because I will need to repeat this process when I install another 2 drives for RAID1 for another purpose – I need to get this down right.

Best Answer

What you have is evidently not hardware RAID but software RAID with a BIOS interface, often called fakeRAID. The main job of putting the disks in an array is done by the Windows driver. Related reading: How do I differentiate “fake RAID” from real RAID?

There are two advantages to hardware RAID over software RAID: it's independent of the operating system (which is only useful if you dual-boot), and it can perform better in some configurations (essentially those that require a cross-drive checksum, which is not the case of RAID-1). Software RAID has the advantage of being independent of the hardware (you can take your disks out and plug them into another machine running the same OS) and tends to come with better tools.

So, forget about your motherboard's fake RAID and use Linux's software RAID. The main tool you'll need to use is mdadm. Create a RAID-1 volume encompassing the two disks, then create partitions on that volume. Something like:

mdadm --create /dev/md0 -l 1 -n 2 /dev/sda /dev/sdb
fdisk /dev/md0

Related Solutions

Linux – How to show available disks on a Linux system

You can use pipes to redirect the output

 fdisk -l | grep ^Disk | awk -F:  '{ print $1 }' |  awk -F" "  '{ print $2 }'

fisk -l: get the full disk output
grep Disk: filter the line starting with Disk
awk -F: '{ print $1 }': get first part where the separator is ":"
awk -F" " '{ print $2 }': get the second part where the separator is space

Lvm – Linux Software RAID1: How to boot after (physically) removing /dev/sda? (LVM, mdadm, Grub2)

You need to install GRUB to the MBR of both drives, and you need to do it in a way that GRUB considers each disk to be the first disk in the system.

GRUB uses its own enumeration for disks, which is abstracted from what the Linux kernel presents. You can change which device it thinks is the first disk (hd0), by using a "device" line in the grub shell, like so:

device (hd0) /dev/sdb

This tells grub that, for all subsequent commands, treat /dev/sdb as the disk hd0. From here you can complete the installation manually:

device (hd0) /dev/sdb
root (hd0,0)
setup (hd0)

This sets up GRUB on the first partition of the disk it considers to be hd0, which you've just set as /dev/sdb.

I do the same for both /dev/sda and /dev/sdb, just to be sure.

Edited to add: I always found the Gentoo Wiki handy, until I did this often enough to commit it to memory.

Best Answer

Related Solutions

Linux – How to show available disks on a Linux system

Lvm – Linux Software RAID1: How to boot after (physically) removing /dev/sda? (LVM, mdadm, Grub2)

Related Topic